Arrangement including an electronic flash tube

ABSTRACT

The invention relates to an arrangement including an electronic flash tube which is fed from a previously charged main capacitor. In a known arrangement a so-called quench tube is arranged across the flash tube. This quench tube is ignited when sufficient light energy is emitted by the flash tube. A drawback of the known arrangement is that the main capacitor becomes completely discharged when this quench tube is ignited. The invention provides a solution in which the shunt across the flash tube also includes a capacitor. The result of this simple step is that the main capacitor is not completely discharged. This has the advantage that the capacitor can be recharged rapidly again for taking the next flash photograph and that the device serving for the supply of this main capacitor is less rapidly exhausted.

United States Patent 1 [111 3,716,753 Exner [451 Feb. 13, 1973 [S4] ARRANGEMENT INCLUDING AN Primary Examiner-Roy Lake ELECTRONIC FLASH TUBE Assistant ExaminerLawrence J. Dahl [75] Inventor: Emil August Exner, Nijmegen, Atwmey Frank Tnfan Netherlands [57] ABSTRACT [73] Assignee: U.S. Philips Corporation, New I York, The invention relates to an arrangement including an electronic .flashtube which is fed from a previously 122] Flled: 1970 charged main capacitor.

1211 Appl' 86,516 In a known arrangement a so-called quench tube is arranged across the flash tube. This quench tube is ig- [30] Foreign Application Priority Data nited when sufficient light energy is emitted by the Nov. 8, 1969 Netherlands 6916883 flash tube A diawback the known arrangement that the mam capacitor becomes completely 52 U.S. Cl ..31s/241 P, 315/159, 315/241 R dscharged when quench tube [51] Int. Cl. ..H05b 37/00 The invention provides a solution in which the shunt Field Of Search 241 241 38 across the flash tube also includes a capacitor. The result of this simple step is that the main capacitor is 1 1 Reierences Ciled not completely discharged. This has the advantage that the capacitor can be recharged rapidly again for UNITED STATES PATENTS taking the next flash photograph and that the device 3,340,426 9/1967 Elliot 315/241 P serving for the supply of this main capacitor is less 3,517,255 6/1970 Hoffer et al..... ....315/241 R rapidly exhausted, 3,541,386 11/1970 Ackermann ..315/159 X 9 Claims, 1 Drawing Figure ARRANGEMENT INCLUDING AN ELECTRONIC FLASH TUBE This invention relates to an arrangement including an electronic flash tube which is ignited from a previously charged main capacitor and in which a supply branch including said tube is shunted by a circuit provided with at least a controlled switching element and in which a control circuit of the switching element includes a photosensitive member, all this in a manner such that the flash duration is influenced as a function of the brightness of an object exposed by the tube.

A known arrangement of the above-mentioned kind is described, for example, in German published application (Auslegeschrift) no. 1,288,428. In an embodiment of this German Auslegeschrift a quench tube is incorporated as a switching element. The method carried out with this known arrangement is as follows: First the flash tube is rendered conducting by an auxiliary electrode so that flashing commences. When, after some time, sufficient light has been reflected by an object to be photographed, the switching element will be rendered conducting through the photosensitive member in the control circuit of this switching element so that the flash tube is extinguished. This flash tube is extinguished earlier as the illuminated object is closer and/or has a lighter color. A drawback of the abovementioned known arrangement is that substantially the entire charge of the main capacitor flows away when the quench tube is actuated. This means that this main capacitor must be recharged to make the arrangement ready for taking the next flash photograph. This is a drawback.

An object of the present invention is to provide an arrangement in which the charge of the main capacitor is used more effectively.

According to the invention an arrangement including an electronic flash tube from a previously charged main capacitor and in which a supply branch including said tube is shunted by a circuit provided with at least a controlled switching element and in which a control circuit of the switching element includes a photosensitive member, all this in a manner such that the flash duration is controlled as a function of the brightness of an object exposed by the flash tube, is characterized in that the circuit shunting the tube incorporates an auxiliary capacitor so that the main capacitor still retains a residual charge after flashing.

An advantage of the above-mentioned arrangement is that, when the switching element is actuated, the auxiliary capacitor is charged to substantially the same voltage value as the instantaneous voltage value of the main capacitor. The voltage on the main capacitor will then not decrease below the minimum operating voltage of the flash tube because the bridge across the flash tube is not a real short circuit, since it includes a capacitor.

To generate the next flash, it is desired to start again from a substantially discharged auxiliary capacitor. The auxiliary capacitor may be discharged, for example, through a resistor but this is not necessary.

The controlled switching element may be, for example, a quench tube. The controlled switching element may be alternatively a controlled semiconductor switching element, for example, a thyristor.

In an advantageous embodiment of an arrangement according to the invention, in which the switching element is a thyristor and the shunt circuit also includes a resistor, the thyristor is shunted by a circuit which includes at least a diode whose pass direction is opposite to that of the thyristor and in which the condition is satisfied that C /C C times the initial voltage of the main capacitor is larger than the minimum operating voltage of the flash tube, in which: C is the capacitance of the main capacitor and C is the capacitance of the auxiliary capacitor in the series circuit shunting the flash tube.

An advantage of this preferred embodiment is that the voltage on the auxiliary capacitor is generally lower at the instant of extinguishing the flash tube than it would be without the above-mentioned diode shunt so that the auxiliary capacitor is discharged earlier and the arrangement is therefore sooner available for generating the next flash. In fact, in this preferred embodiment the auxiliary capacitor will first be charged after the switching element has been rendered conducting, the voltage across the capacitors still being above the minimum operating voltage of the flash tube after this distribution of the charge of the main capacitor over,

the main and auxiliary capacitors. As a result thereof the main capacitor will supply further energy to the flash tube and the auxiliary capacitor will pass a current through the diode to the flash tube. By adjusting the control circuit of the thyristor in the shunt circuit at a slightly earlier instant, it can be achieved that the flash from the flash tube radiates the exact quantity of light in spite of this current flowing through the diode. The mentioned light quantity is a quantity which is desirable in view of the distance between the camera and the object and the color of the object. Also in this preferred embodiment the residual charge of the main capacitor will not drop below that of the minimum operating voltage of the flash tube.. In practical cases this voltage is approximately Volts.

In order that the invention may be readily carried into effect, an embodiment thereof will now be described in detail, by way of example, with reference to the sole FIGURE of the accompanying diagrammatic drawing showing an electric circuit of an arrangement according to the invention. In this drawing, the reference numerals l and 2 denote connecting terminals which are intended to be connected to an arrangement for DC charging a capacitor 3 which is arranged between terminals 1 and 2. In the FIGURE, the reference numeral 4 denotes an electronic flash tube. This tube is connected in series with the primary winding 5 of a transformer. The series arrangement of flash tube 4 and transformer winding 5 together shunt the capacitor 3. The flash tube 4 together with the transformer winding 5 are shunted by a series arrangement of a resistor 6, a thyristor 7 and a capacitor 8. The thyristor 7 is shunted by the series arrangement of a resistor 9 and a diode 10. The capacitor 8 is shunted by a resistor 11 having a high resistance. The reference numeral l2 denotes the secondary winding of the transformer whose primary winding is denoted by reference numeral 5. One end of this transformer winding 12 is connected to a diode 13 and a capacitor 14. The other end of capacitor 14 is connected to the other end of the transformer winding 12. The capacitor 14 is shunted by a series arrangement of a photosensitive resistor 15, a variable adjusting resistor 16 and a subsequent variable adjusting resistor 17. The variable resistor 17 is in turn shunted by a capacitor 18. One end of the capacitor 18 is connected to the control electrode of the thyristor 7 and the other electrode of capacitor 18 is connected to a junction of thyristor 7 and capacitor 8. Furthermore, the FIGURE diagrammatically shows a control electrode of the flash tube 4 which is denoted. by the reference numeral 19. This control electrode is connected to a terminal 20. In addition a main electrode of the tube 4 is connected to a connecting terminal 21.

The operation of the circuit described is as follows. First the capacitor 3 is DC charged through the terminals 1 and 2 by an arrangement not shown. Subsequently, when taking a photograph and by simultaneously closing the camera contact a voltage is applied between terminals 20 and 21 by an arrangement not further shown. As a result a voltage is applied to the trigger electrode 19 so that the flash tube 4 is ignited. As a result a current will flow through the primary winding 5 of the transformer which is connected in series with the tube 4. Consequently, a voltage will also be generated in the secondary winding 12 so that capacitor 14 will be charged. The tube 4 lights an object while part of the light reflected by this object is incident on the photosensitive resistor 15. As a result the resistance of this cell 15 will decrease. Due to the decreased resistance of cell 15 a current will then flow from capacitor 14 to capacitor 18. When a certain quantity of light is incident on the cell 15 the capacitor 18 will have received a voltage so high that the thyristor 7 is triggered. In the then conducting state of the thyristor a current will flow through the shunt circuit 6, 7, 8. This current will flow for such a period until the voltage on capacitors 3 and 8 has become substantially equal. The arrangement is proportioned in such a manner that the charge which is then distributed will generally be such that the voltage on capacitor 3 is still higher than the minimum operating voltage of the tube 4. The result thereof is that two current circuits are now present, in one of which the main capacitor 3 will still further be discharged across tube 4. The second current circuit is constituted by capacitor 8, resistor 9, diode 10, resistor 6 and the tube 4. The second current flows through the tube 4 in the last mentioned circuit in the direction of current flow determined by diode 10. When the charge of the two capacitors has decreased in this way to the minimum operating voltage of tube 4, this tube will be extinguished. The resistor 1 1 in the circuit is a resistor which shunted the capacitor 8. This resistor is provided to ensure that the capacitor 8 does not remain in its charged state when a flash is effected and when this flash is extinguished. In fact, this would be detrimental for taking further flash photographs immediately thereafter. if desired, a switching element may be arranged in the position of resistor 1 l.

in a practical case the capacitor 3 had a capacitance of 600 F. in that case also the capacitance of the capacitor denoted by the reference numeral 8 was 600 F. The capacitance of capacitor 14 and capacitor 18 was 20 1F each. Resistor l6 and resistor 17 were both variable between approximately 50 and S Ohms. Resistor 6 had a value of approximately 0.28 Ohms. Resistor 9 had a value of approximately Ohms and resistor 11 had a value of approximately 50 k. Ohms. The starting point was an initial voltage on the capacitor 3 of approximately 350 Volts direct voltage. The minimum operating voltage of the electronic flash tube 4 was approximately Volts. Consequently, in this case the condition is satisfied that C,/C +C times the initial voltage of the capacitor 3 is larger than the minimum operating voltage of the tube, for 600/600+600 X 350 Volts is larger than 80 Volts.

In the arrangement according to the invention, the main capacitor 3 is not completely discharged, that is to say, a residual charge of approximately 80 Volts remains so that a battery may charge this capacitor rapidly to 350 Volts again. in addition, the battery which is used for charging this capacitor 3, or any other device which serves for charging capacitor 3, need supply less energy so that this battery or any other device need not be replaced too soon.

What is claimed is:

1. A flash tube control circuit comprising, a supply capacitor connected across the flash tube, means for charging the capacitor to a given voltage, a controlled switching element, an auxiliary capacitor, means connecting said switching element in series circuit with said auxiliary capacitor in a shunt circuit across the flash tube, a photosensitive member arranged to respond to the flash tube light reflected by an object exposed thereto, and a control circuit including said photosensitive member coupled to a control electrode of the switching element in a manner such that the triggering of the switching element and hence the flash duration is controlled as a function of the brightness of the exposed object, said shunt circuit being arranged to prevent the complete discharge of the supply capacitor during a flash cycle.

2. A control circuit as claimed in claim 1 wherein the switching element comprises a thyristor and the shunt circuit also includes a resistor, a diode, means connecting the diode in shunt circuit with the thyristor with the diode pass direction opposite to that of the thyristor, said capacitors being chosen so that (l /C C times the given voltage of the supply capacitor is larger than the minimum operating voltage of the flash tube in which:

C is the capacitance of the supply capacitor and C is the capacitance of the auxiliary capacitor.

3. A control circuit as claimed in claim 1 wherein said switching element comprises a thyristor, said circuit further comprising a diode connected across said thyristor with opposite polarity so. as to provide a discharge path for said auxiliary capacitor that includes the flash tube.

4. A control circuit as claimed in claim 3 further comprising a resistor connected in series with said thyristor and said auxiliary capacitor across the flash tube and in series with said diode and flash tube in the auxiliary capacitor discharge path.

5. control circuit as claimed in claim 1 wherein said control circuit includes means responsive to the flow of current in the flash tube to derive a control signal for said switching element.

6. A control circuit as claimed in claim 1 wherein said shunt circuit further comprises a resistor connected in series with the switching element and the auxiliary capacitor in the order named, a diode connected in parallel with the switching element with opposite polarity, a second resistor connected across said auxoperating voltage of the flash tube wherein: C is the capacitance of the supply capacitor and C is the capacitance of the auxiliary capacitor.

8. A control circuit as claimed in claim 2 wherein C is equal to C 9. A control circuit as claimed in claim 1 further comprising a resistor connected in shunt with the auxiliary capacitor. 

1. A flash tube control circuit comprising, a supply capacitor connected across the flash tube, means for charging the capacitor to a given voltage, a controlled switching element, an auxiliary capacitor, means connecting said switching element in series circuit with said auxiliary capacitor in a shunt circuit across the flash tube, a photosensitive member arranged to respond to the flash tube light reflected by an object exposed thereto, and a control circuit including said photosensitive member coupled to a control electrode of the switching element in a manner such that the triggering of the switching element and hence the flash duration is controlled as a function of the brightness of the exposed object, said shunt circuit being arranged to prevent the complete discharge of the supply capacitor during a flash cycle.
 1. A flash tube control circuit comprising, a supply capacitor connected across the flash tube, means for charging the capacitor to a given voltage, a controlled switching element, an auxiliary capacitor, means connecting said switching element in series circuit with said auxiliary capacitor in a shunt circuit across the flash tube, a photosensitive member arranged to respond to the flash tube light reflected by an object exposed thereto, and a control circuit including said photosensitive member coupled to a control electrode of the switching element in a manner such that the triggering of the switching element and hence the flash duration is controlled as a function of the brightness of the exposed object, said shunt circuit being arranged to prevent the complete discharge of the supply capacitor during a flash cycle.
 2. A control circuit as claimed in claim 1 wherein the switching element comprises a thyristor and the shunt circuit also includes a resistor, a diode, means connecting the diode in shunt circuit with the thyristor with the diode pass direction opposite to that of the thyristor, said capacitors being chosen so that C1/C1 + C2 times the given voltage of the supply capacitor is larger than the minimum operating voltage of the flash tube in which: C1 is the capacitance of the supply capacitor and C2 is the capacitance of the auxiliary capacitor.
 3. A control circuit as claimed in claim 1 wherein said switching element comprises a thyristor, said circuit further comprising a diode connected across said thyristor with opposite polarity so as to provide a discharge path for said auxiliary capacitor that includes the flash tube.
 4. A control circuit as claimed in claim 3 further comprising a resistor connected in series with said thyristor and said auxiliary capacitor across the flash tube and in series with said diode and flash tube in the auxiliary capacitor discharge path.
 5. A control circuit as claimed in claim 1 wherein said control circuit includes means responsive to the flow of current in the flash tube to derive a control signal for said switching element.
 6. A control circuit as claimed in claim 1 wherein said shunt circuit further comprises a resistor connected in series with the switching element and the auxiliary capacitor in the order named, a diode connected in parallel with the switching element with opposite polarity, a second resistor connected across said auxiliary capacitor, means for applying a trigger voltage to a control electrode of the flash tube and wherein said control circuit further comprises a transformer with a primary winding in series with the flash tube and a secondary winding coupled to the photosensitive member and to the control electrode of the switching element.
 7. A control circuit as claimed in claim 3 wherein the capacitors are chosen so that C1/C1 + C2 times the given voltage of the supply capacitor exceeds the minimum operating voltage of the flash tube wherein: C1 is the capacitance of the supply capacitor and C2 is the capacitance of the auxiliary capacitor.
 8. A control circuit as claimed in claim 2 wherein C1 is equal to C2. 